1. Technical Field
The present invention relates to carbon nanofibers, and more particularly, to a method capable of preparing metal oxide-containing porous carbon nanofibers having a high specific surface area by changing the composition of a spinning solution, which is used in a process of preparing carbon nanofiber by electrospinning, and to metal oxide-containing porous carbon nanofibers prepared by the method, and carbon nanofiber products comprising the same.
2. Description of the Related Art
With the advent of the ubiquitous era, the development of environmentally friendly and highly efficient materials is necessary to achieve the miniaturization of various electronic telecommunication devices and the practical use of electric vehicles.
In the 21st century, high-tech small-size mobile power systems are needed and the size and weight of these systems must be reduced. In connection with this requirement, highly functional carbon materials have received attention and studies on the development thereof have been made.
Among highly functional carbon materials, activated carbon has a high specific surface area, but the pore structure thereof is very complex, making it difficult to reproduce adsorption and desorption rates. In the case of activated carbon fibers, the fine pores are exposed to the outside, but have a diameter on the micrometer scale, and thus exhibit limited capacity and reaction rate, which make it difficult to use the activated carbon fibers as energy storage media.
On the other hand, carbon nanofibers have a uniform pore distribution and high specific surface area, compared to activated carbon, and can be prepared in the form of paper, felt or nonwoven fabric. Thus, the carbon nanofibers can provide electrode active materials having better performance. In addition, carbon nanofibers having nanographite structures have a relatively large specific surface area and shallow pore depth and include fine pores having a size of 1-2 nm, which show high adsorption and desorption rates. In addition, these carbon nanofibers have a uniform pore structure and a narrow pore size distribution, and thus show fast selective adsorption and desorption even when the energy slightly changes. Thus, these carbon nanofibers have very excellent energy storage properties.
In the carbon nanofiber preparation methods known to date, a chemical activation process should be carried out after a heat-treatment process (stabilization or carbonization process) in order to prepare porous carbon nanofibers. Specifically, after the carbonization process has been carried out at high temperature, the chemical activation process is generally carried out by mixing carbon nanofibers with potassium hydroxide (KOH) or sodium hydroxide (NaOH) at high temperature and then heat-treating the mixture at high temperature. However, in the carbon nanofiber preparation method including the chemical activation process that employs a salt, there are problems in that it is difficult to carry out the process continuously and to prepare large amounts of porous carbon nanofibers, because heat-treatment is carried out after the uniform mixing of the carbon nanofibers with the salt, and a process of removing the added salt is required after the activation process.
Methods of preparing PAN-based carbon nanofibers and pitch-based carbon nanofibers using an electrostatic spinning technique are disclosed in Korean Patent Laid-Open Publication Nos. 2002-0008227 and 10-2003-0002759, respectively. In these patent documents, PAN-based carbon nanofibers are prepared by electrospinning a PAN solution and stabilizing, carbonizing and activating the spun fibers. However, the PAN-based carbon fibers have a low specific surface area and low electrical conductivity, and do not sufficiently exhibit the performance of electrodes for double layer supercapacitors. Meanwhile, the fiber diameter of the pitch-based carbon nanofibers prepared by the above method is disadvantageously very large because the ability to be spun is low.
In recent years, in order to develop electric double-layer capacitors into power supply devices for electric vehicles, which require high output and high capacity properties, studies have been done into the use of carbon nanomaterials as electrodes for electric double-layer capacitors.
Electric double-layer capacitors are devices that use charges accumulated in an electric double layer formed between a solid electrode and an electrolyte, and have received the attention of various fields. Particularly, capacitors have low energy density compared to cells, but exhibit excellent power density properties and are almost semi-permanent, and thus are expected to be used in various fields. Thus, in the case of electrochemical capacitors, research and development has been actively carried out to improve both the energy density and power density of the capacitors. Among energy storage devices, the performance of supercapacitors greatly varies depending on the material and the fabrication technology, and thus it is very important to fabricate supercapacitors having high energy density using newly developed materials.
Electrolytes that are used in electric double-layer capacitors (ELDCs) are largely classified into water soluble electrolytes, organic solvent-based electrolytes and solid electrolytes. Because the potential difference of EDLC unit cells during use is determined by the electrolyte, the choice of electrolyte is very important. Generally, aqueous electric double-layer capacitors have a shortcoming in that their operating voltage is as low as 1.0 V or lower and the amount of energy that can be stored therein is limited. When an organic solvent electrolyte is used to overcome these shortcomings, the capacitor can be used at high cell voltage, and thus can store a large amount of energy.
However, when the organic solvent electrolyte is used, there is the disadvantage of increasing the internal resistance of the capacity so that the charge/discharge characteristics are poor compared to those of aqueous double-layer capacitors. Nevertheless, in this case, a capacitor can be obtained, which has a high potential difference during use and a high energy density that increases in proportion to the square of the voltage during use. This capacitor can be used in a wide temperature range of −25˜85° C., can have high breakdown voltage and can be miniaturized.
Electrodes for EDLCs are made mainly of activated carbon materials, which have large specific surface areas and are electrically stable and are also highly electrically conductive. Specifically, the electrodes are made mainly of activated carbon materials or activated carbon fibers prepared by activating materials from coal or petroleum pitch, phenol resin, woody and carbon precursor polymers as starting materials using an oxidative gas or an organic salt at a temperature lower than 1200° C.
Methods for increasing the energy density of electric double-layer capacitors include methods of fabricating hybrid capacitors (10-20 Wh/kg) using the asymmetric electrode technique, and methods of fabricating electric double-layer capacitors (20-40 Wh/kg) from high-capacity activated carbon prepared using an alkaline activator (The Korean Institute of Electrical and Electronic Material Engineers, 17, 1079 (2004); Mat. Res. Soc. Proc., San Francisco, Calif., 397 (1995); J. Electrochem. Soc., 148, A930 (2001); Electrochem., 69, 487 (2001); Carbon, 43, 2960 (2005)). In the case of the latter, the graphite carbon material is heat-treated with an alkaline material (KOH, NaOH or K2CO3) at a high temperature of 700˜900° C., and the activated material has a capacitance of about 30-50 F/mL per electrode volume at 3.5 V. However, it was reported that activated carbon materials prepared by the alkaline activation method have problems in that they should be heat-treated during their preparation, corrode containers, the characteristics thereof are deteriorated due to charge/discharge cycles and the production cost thereof is high.
Meanwhile, activated carbon fibers which are currently produced and sold are mainly prepared by obtaining fibers from precursors using an expensive melt-spinning or melt-blown spinning apparatus, and then stabilizing and carbonizing or activating the fibers. However, the process used in this preparation method is complex, and the diameter of the fibers is large, making it difficult to effectively increase the specific surface per volume.
In addition, when the prepared fibers are to be used as electrode active materials, a process of crushing the fibers and adding a binder or a conductive material thereto should be carried out. Furthermore, when the fibers are used in the form of woven fabric as an electrode, the density of the electrode is low because of the large diameter of the fibers, and thus the high-speed charge/discharge or high-output properties are deteriorated.
Accordingly, there is a need to develop carbon nanofibers which overcome the above-described problems.